1. Field of the Invention
The present invention relates to a projection aligner for use in the process of manufacturing LSIs.
2. Description of the Related Art
FIG. 20 shows a conventional projection aligner. A fly-eye lens 3 is disposed diagonally to a lamp house 1. A mirror 2 is disposed between the lens 3 and the house 1. An aperture member 4 is positioned in front of the fly-eye lens 3. Condenser lenses 5 and 6, a mirror 7, and an exposure mask 8 on which a desired circuit pattern is formed are arranged along an optical path. A wafer 10 is situated in front of the mask 8, and a projecting lens system 9 is disposed between the mask 8 and the wafer 10.
As shown in FIGS. 21 and 22, the aperture member 4 has a disk-like configuration with a circular opening 4a at the center thereof.
Light emanating from the lamp house 1 reaches the fly-eye lens 3 through the mirror 2, and is split into light beams which pass through lenses 3a, of the fly-eye lens 3. The light transmitted through the respective lenses 3a pass through the opening 4a of the aperture member 4, the condenser lens 5, the mirror 7 and the condenser lens 6, and then irradiate an exposure zone of the mask 8. The light beams transmitted through the lenses 3a of the fly-eye lens 3 are superposed on each other on the surface of the mask 8, and thus the beams irradiate uniformly the surface of the mask 8. In this way, the light beams pass through the mask 8 and reach the wafer 10 through the projecting lens system 9, whereby the circuit pattern is imaged on the surface of the wafer 10.
It is known that the minimum resolution R of such a projection aligner is proportional to .lambda./NA, where .lambda. is the wavelength being used and NA is the numerical aperture of the optical system. Thus, the optical system has hitherto been designed so that the numerical aperture is increased to improve the resolution of the projection aligner. In recent years the improved resolution copes with a higher degree of integration of LSIs.
It is also known that as the numerical aperture of an optical system increases, the minimum resolution R decreases and the depth of focus (DOF) of the projection aligner also decreases even more than the resolution R does. The DOF is proportional to .lambda./NA.sup.2. For this reason, in the conventional projection aligner, the DOF decreases with an increase in the resolution, and the accuracy in transferring transcribing the circuit pattern deteriorates.